FIG. 8 shows an example (Patent Document 1) of a prior-art combustion chamber shape in a direct injection type diesel engine. A combustion chamber (cavity) 106 in a shallow dish recessed shape is formed in a top wall 102 of a piston 101. On a wall face of the combustion chamber 106, a mountain portion 104 spreading from a center of the combustion chamber 106 and sloping gently to a vicinity of an outer peripheral end portion of the combustion chamber is formed and a saucer portion 105 seamlessly connected to a skirt end of the mountain portion 104 and rising in an arc shape is formed. A fuel injection valve 103 is mounted to a cylinder head 110 so that a center O1 of a nozzle hole is positioned on a cylinder center line C1 and injects fuel in a conical shape toward the saucer portion 105 of the combustion chamber 106 at a predetermined nozzle hole angle α.
In a structure in FIG. 8, about a half of fuel spray directly collides with a slope of the mountain portion 104 to thereby suppress fuel and a NOx generation amount. However, during no-load operation, a proportion of the fuel adhering to the slope of the mountain portion 104 increases due to reduction in injection pressure to thereby increase black smoke.
Patent Document 1: Japanese Patent Application Laid-open No. 5-106442